1. Field of the Invention
The invention relates to an optical device which is used for an image display device or the like, and a multisurface reflector which is used for this optical device.
2. Description of Related Art
Conventionally, a liquid crystal projector device of the projection type using a liquid crystal cell is known as an image display device for displaying large images. In this image display device, an optical device is necessary which illuminates the optical images formed by the liquid crystal cell with light with high intensity. The efficiency of this optical device greatly affects the image quality of the projection images in the image display device. Therefore, there is a need for an optical device with high efficiency and color reproducibility, which exhibits uniformity of the illumination intensity of the overall surface in the area to be illuminated and also advantageous color uniformity.
FIG. 6 schematically depicts a conventional example optical device arrangement. This known optical device is composed of a parabolic reflector 71 and a light source lamp 72 which is located along a center axis L of this parabolic reflector 71. A liquid crystal cell 78 is located on area R to be illuminated by the above described optical device.
In this optical device, light which is emitted from an emission part H of the light source lamp 72 is reflected by parabolic reflector 71. In this way, a light beam is formed which is essentially parallel to the center axis L of parabolic reflector 71 and which irradiates the liquid crystal cell 78 which is located on the illumination area R.
By this optical device, the light emitted from the light source lamp 72 is, for the most part, focussed by means of parabolic reflector 71, and is emitted as parallel light beam onto illumination area R. Therefore, a high utilization factor of the light can be obtained.
However, in the above described optical device, the light beam formed by means of the parabolic reflector 71 in the vicinity of its center has a high light flux density, while it has a low light flux density on its periphery. Therefore, the total surface of the area R to be illuminated cannot be illuminated with a uniform illumination intensity.
To improve the uniformity of the illumination intensity on the overall surface of the area R to be illuminated, a means is known by which the surface of the bulb of a light source lamp is subjected to frost treatment. On the surface of the bulb subjected to frost treatment, the light is diffused from the emission part. In doing so, therefore, it is considered disadvantageous that the intensity of the light which irradiates the area to be illuminated decreases considerably.
On the other hand, as another means for improving the uniformity of the illumination intensity on the overall surface of the area to be illuminated, a means using an integrator lens is known (see, U.S. Pat. No. 2,186,123).
FIG. 7 shows, in schematic form, a conventional example of the arrangement of an optical device B using an integrator lens. In this optical device, a first lens plate 73, on which several lens elements 74 are located, is arranged in front of parabolic reflector 71 on a plane perpendicular to center axis L of the above described parabolic reflector. Between this first lens plate 73 and illumination area R, a second lens plate 75 is arranged in front of the first lens plate 73 such that it is spaced parallel to and from the first lens plate 73. On second lens plate 75, there are numerous lens elements 76, each of which corresponds to a lens element 74 on the first lens plate 73. Specifically, second lens plate 75 is arranged such that respective lens element 76 is positioned in the respective focal point of corresponding lens element 74 of first lens plate 73.
In this optical device, the light beam which is parallel to center axis L and which was formed by means of the parabolic reflector 71 is divided by respective lens element 74 of first lens plate 73 according to above described respective lens element 74 and this divided light is converged by corresponding respective lens element 76 on second lens plate 75. In this case, a real image of the emission part H of light source lamp 72 is formed on the respective lens element 76 of the second lens plate 75. The respective light which was converged by lens element 76 with second lens plate 75 is superimposed by the lens element 76 in the state in which the real image of emission part H is enlarged, in the same area R to be illuminated.
In the light divided by first lens plate 73, the light flux density in the vicinity of the center has a small difference from the light flux density on the periphery due to the above described optical device. Furthermore, by means of second lens plate 75, the respective divided light is superimposed in the same area R to be illuminated. In this way, high uniformity of the illumination intensity can be obtained on the overall surface of above described illumination area R.
In the above described optical device the following disadvantages arise:
(1) Two lens plates are needed, i.e., the first lens plate 73 and the second lens plate 75. It is also necessary that the lens plates be arranged at a distance from one another. Therefore, the optical device has a larger dimension in the direction of the optical axis (center axis L of the parabolic reflector 71). In this way, the overall device becomes large. PA1 (2) The size of the real image which is formed on respective lens element 76 of second lens plate 75 is determined by the size of emission part H and the distance between the parabolic reflector 71 and the respective lens element 76 of the second lens plate 75. This means that the larger the emission part H of light source lamp 72 and the greater the distance between parabolic reflector 71 and respective lens element 76 of second lens plate 75, the larger becomes the real image formed. PA1 (3) The equilibrium of the spectral energy for the light emitted from entire emission part H cannot be maintained since the light emitted from the peripheral area of emission part H of light source lamp 72 is not used by second lens plate 75, as was described above. Therefore, no light was obtained which has the expected spectral distribution. In particular, in the case in which an image display device which displays color images is used, an image with the expected hue cannot be projected. This is described specifically in the following:
As light source, a metal halide lamp of the short arc type is generally used for lamp 72 because it has high radiant efficiency, high uniformity of the spectral energy distribution, and good color reproduction. The emission part of this metal halide lamp, for example, has a length from 3 to 4 mm, a width of 2 to 3 mm, a roughly cylindrical shape, and is relatively large. Furthermore, the distance between the parabolic reflector 71 and the respective lens element 76 of the second lens plate 75 is rather large, because first lens plate 73 must be placed between parabolic reflector 71 and second lens plate 75.
The real image of emission part H is formed on respective lens element 76 of second lens plate 75. However, light which corresponds to a part of the real image of above described emission part H which diverges from the aperture of lens element 76, i.e., light which is emitted by the peripheral area of emission part H, cannot be used, since conventionally the aperture of lens element 76 is smaller than the real image of emission part H. Therefore, a high utilization factor of the light cannot be obtained.
On the other hand, in the case in which the second lens plate consists of lens elements which have larger openings than the formed real image of emission part H, a high light utilization factor can be obtained. However, here, the dimensions of the entire second lens plate become large. The optical device, therefore, becomes large in the direction which intersects the optical axis. Furthermore, the light which emerges from the lens elements in the peripheral area of the second lens plate has a large irradiation angle with respect to area R to be illuminated. Therefore, a projection lens is needed with a small F number. However, it is difficult to form a low cost image display device with a small shape because a projection lens with a small F number has a large effective aperture and high costs.
FIG. 8 is a schematic of an arrangement of a liquid crystal projector device of the projection type for display of color images which has the optical device B with the configuration shown in FIG. 7. In this liquid crystal projector device, the light which emerges from each lens element 76 of the second lens plate 75 in the optical device 70 of the configuration shown in FIG. 7 is reflected by total reflex mirror 80a, then passes through a UV-IR-cut filter 81 which screens out ultraviolet radiation and infrared radiation reaches mirror 82. Mirror 82 is provided with a multilayer color separation film and the infrared radiation reaching this mirror 82 is separated into a red component, a green component and a blue component.
The light with the red component is emitted via total reflex mirror 80b and condenser lens 84a onto a liquid crystal cell for red 85 which is located on an illumination area R1. On the other hand, the light with the green component and the light with the blue component are separated by means of a mirror 83 having a multilayer color separation film, the light with the green component being emitted via condenser lens 84b onto a liquid crystal cell for green 86 which is located on area illumination area R2. The light with the blue component is emitted via condenser lens 84c onto a liquid crystal cell for blue 87 which is located on an illumination area R3.
The light which emerges from respective lens element 76 of second lens plate 75 is separated in this way into the red component, the green component, and the blue component and then superimposed at liquid crystal cell for red 85, at liquid crystal cell for green 86, and liquid crystal cell for blue 87, respectively.
The light with the red component and the light with the green component are combined by mirror 88 with a multilayer color synthesis film. Furthermore, this light is combined with the light with the blue component by mirror 89 which also has a multilayer color synthesis film. This combined light is emitted via projection lens 90 onto a suitable screen which is not shown in the drawing. In this way, a color image is projected on the above described screen.
The metal halide lamp used as light source lamp 20 has high uniformity of the spectral energy distribution for light which is emitted from the entire emission part. The spectral distribution of the emitted light is, however, different according to the area of the emission part. This means that, in the center area of the emission part, the light ratio is large due to the mercury line, while in the peripheral area of the emission part, the light ratio with wavelengths that are greater than or equal to 600 nm is large due to molecular emission.
Therefore, if the light which is emitted from the peripheral area of the emission part of the metal halide lamp is lost by second lens plate 75, the red component with wavelengths of greater than or equal to 600 nm decreases in a large ratio, as was described above. The equilibrium of the spectral energy for the second lens plate 75, therefore, cannot be maintained. As a result, an image with the expected hue cannot be projected onto the screen.